1. Field of the Invention
The invention relates to ocular implants, and, in particular, to an implant and method used in the treatment of glaucoma.
2. Background
Intraocular pressure in the eye is maintained by the formation and drainage of aqueous, a clear, colorless fluid that fills the anterior and posterior chambers of the eye. Aqueous normally flows from the anterior chamber of the eye out through an aqueous outflow channel at a rate of 2 to 5 microliters per minute. Glaucoma is a progressive disease of the eye characterized by a gradual increase of intraocular pressure. This increase in pressure is most commonly caused by stenosis or blockage of the aqueous outflow channel, resulting in excessive buildup of aqueous fluid in the eyeball, Other causes include increase in venous pressure outside the eye which is reflected back through the aqueous drainage channels and increased production of aqueous. In a "normal" eye, intraocular pressure ranges from 8 to 21 mm mercury. In an eye with glaucoma, this pressure can range between the so called normal pressures and pressures up to as much as 50 mm mercury. This increase in intraocular pressure produces gradual and permanent loss of vision in the afflicted eye.
Existing corrective methods for the treatment of glaucoma include drugs, surgery, and implants. Pharmacological treatment is prohibitively expensive to a large majority of glaucoma patients. In addition, many people afflicted with the disease live in remote or undeveloped remote areas where the drugs are not readily accessible. The drugs used in the treatment, in particular steroids, often have undesirable side effects and many of the long-term effects resulting from prolonged use are not yet known.
Surgical procedures have been developed in an effort to treat victims of glaucoma. An iridectomy, removal of a portion of the iris, is often used in angle-closure glaucoma wherein there is an occlusion of the trabecular meshwork by iris contact. Removal of a piece of the iris then gives the aqueous free passage from the posterior to the anterior chambers in the eye. A trabeculectomy, opening the inner wall of Schlemm's canal is often performed in cases of developmental or juvenile glaucoma so as to increase the outflow of the aqueous, thereby decreasing intraocular pressure. In adults, a trabeculotomy shunts fluid through a trap-door flap in the eye that performs a valve-like function for the first few weeks after surgery. While often successful, these surgical techniques possess inherent risks associated with invasive surgery on an already afflicted eye. Furthermore, the tissue of the eye can scar over this small area and the eye reverts to the pre-operative condition, thereby necessitating the need for further treatment.
Ocular implants are often used in long-term glaucoma treatment. One early implant was invented by Dr. Anthony Molteno and is described in the paper entitled "Use of Molteno Implants to Treat Secondary Glaucoma" by A. C. B. Molteno and published by Grune & Stratton, Ltd, 1986, pp 211-238, which is hereby incorporated by reference in its entirety. The implant was a small circular plate with a rigid translimbal drainage tube attached thereto. The plate was 8.5 mm in diameter and formed a surface area of 48 mm.sup.2. This early Molteno implant was sutured to the sclera in the anterior segment of the eye at the limbus and the drainage tube was inserted into the anterior chamber of the eye. Once implanted, the body forms scar tissue around this plate. Increased pressure causes the tissues above the plate to lift and form a bleb into which aqueous flows from the anterior chamber via the drainage tube. Many problems occurred with the early Molteno device. The bleb that formed around the plate, even through it was small in surface area on the sclera, formed a very tall bleb which resulted in Dellen formation (sterile corneal ulcers). The implant had to be removed in another surgery to cure the ulcers. Further, as indicated at page 220 of Dr. Molteno's paper, his early device did not reduce the intraocular pressure enough to treat the glaucoma without the use of additional medications. Dr. Molteno redesigned his implant for insertion into the posterior segment of the eye to avoid the problems with his early anterior device.
The redesigned Molteno implant is disclosed in U.S. Pat. No. 4,457,757 entitled "Device for Draining Aqueous Humor," which is hereby incorporated by reference in its entirety. This implant is commercially available as the Molteno.TM. Seton Implant and also referred to as the long tube molteno implant. As indicated at page 221 of Dr. Molteno's paper, the long tube Molteno implant has been used exclusively by Dr. Molteno since 1973. The implant comprises a flexible drainage tube connected to one or more rigid plate reservoirs. The plates are shaped to conform to the curvature of the eye. The long tube Molteno implant is disadvantageous as the plates are formed of a rigid plastic which makes insertion beneath the eye tissue difficult and time-consuming. The plates are 13 mm in diameter and therefore the bleb formation area is at least 134 mm.sup.2. The reservoir plate is placed under Tenon's capsule in the posterior segment of the eye and sutured to the sclera. The drainage tube is implanted into the anterior chamber through a scleral flap. A second plate can be passed under or over the superior rectus muscle also in the posterior segment of the eye and sutured to the sclera. In the redesign of the Molteno implant, Dr. Molteno not only moved the bleb forming part, or plate, of the implant back to the posterior segment of the eye, he also increased the bleb formation area from at least 48 to at least 134 mm.sup.2, because it was believed that there was not enough room to form a large bleb in the anterior segment of the eye.
U.S. Pat. No. 4,750,901 issued to Molteno, which is hereby incorporated herein by reference, discloses another glaucoma implant with an elevated peripheral ridge, a subsidiary elevated ridge on the upper surface of the implant and a drainage tube which leads from the upper surface of the plate to the anterior chamber of the eye. This device is also implanted in the posterior segment of the eye under Tenon's tissue, i.e., Tenon's capsule. This Molteno patent discloses that the tube enters the peripheral ridge to a position above the upper surface of the plate and the subsidiary ridge is located around the entrance of the tube. The subsidiary ridge is forced against Tenon's capsule, to create an initial bleb cavity much smaller in area than the total bleb cavity, but both cavities are formed in the posterior segment of the eye. This Molteno patent discloses that the addition of the subsidiary ridge to the upper surface of the plate around the exit of the tube has the effect of providing a pressure sensitive one-way valve effect. The Molteno patent also discloses that, once the eye recovers from the operation, the increased production of aqueous fluid by the eye raises the pressure in the eye and also within the small bleb cavity causing the overlying Tenon's capsule to be lifted slightly, thereby allowing fluid to flow into the entire bleb cavity. In practice however, the Molteno device fails to provide an effective sealing surface with Tenon's capsule and the desired one-way valve effect does not occur.
UK Patent Application 2,160,778 entitled "Aqueous humor drainage device" discloses a similar type of implant device comprising a drainage tube and a drainage body. The tube is fixed to and opens directly onto a surface of the body. The device is sutured to the selera of the eye in the posterior segment of the eye and the tube positioned within the anterior chamber to provide outflow for the aqueous contained therein.
U.S. Pat. No. 4,729,761 discloses a Glaucoma implant with a plate, a separate fluid reservoir, a first tube between the plate and the fluid reservoir and a second tube between the fluid reservoir and the anterior chamber of the eye. The plate is attached to the sclera in the posterior segment of the eye, and a bleb forms around the plate. Despite the more anterior location of the fluid reservoir, the housing around which the bleb is formed is located in the posterior segment of the eye.
Another glaucoma implant device called the Optimed Glaucoma Implant made by Optimed, Inc. of Santa Barbara, Calif. This implant comprises a box valve connected to a drainage tube which extends into the anterior chamber of the eye. The box valve has a dimension of approximately 3 mm.times.2 mm.times.2 mm and has a maximum top surface area of 18 mm.sup.2. The box has a small extension with holes therein to form suture locations to attach the box to the eye. The box contains approximately 180-200 microtubules which are attached to one end of the drainage tube. The microtubules act like a valve to limit the flow of aqueous humor from the anterior chamber. The drainage tube is implanted into the anterior chamber of the eye. The box and housing is sutured to the sclera in the anterior segment close to the limbus. The housing does not constitute a plate or a drainage surface.